Various gases, for example, oxygen, can be separated from air or other feed streams by systems of ion conducting, ceramic membranes. Such ceramic membranes exhibit ion conductivity at temperatures well above 500.degree. C., generally in a range of between about 600.degree. C. and about 1100.degree. C. A central issue surrounding the use of such membranes is that the transport rate of the gas through the membrane must take place at a sufficient rate to make the separation economically attractive.
For instance, ceramic membrane materials useful in separating oxygen, in general, are mixed conductors, which possess both oxygen ion conduction and electronic conduction in either single-phase or dual-phase states. The driving force of the overall oxygen transport rate for the membrane is the different oxygen partial pressure applied across the membrane. Since the membrane is dense and gas tight, the direct passage of oxygen molecules is blocked. Oxygen ions, however, can migrate selectively through the membrane. Dissociation and ionization of oxygen occurs at the membrane cathode surface where electrons are picked up from near surface electronic states. The flux of oxygen ions is charge compensated by a simultaneous flux of electronic charge carriers. When the oxygen ions arrive at the opposite anode surface of the membrane, the individual ions release their electrons and recombine again to form oxygen molecules, which are released in the permeate stream.
The permeation rate through a non-porous ceramic membrane is controlled by two major factors: (1) the solid-state ionic transport rate within the membrane and (2) the ion surface exchange rate on either side of the membrane. The flux of the gas to be separated usually can be increased by reducing the thickness of the membrane, until its thickness reaches a critical value. At above the critical value, the flux is controlled by both the ion surface exchange kinetics and solid state ionic transport rate. Below the critical thickness, the oxygen flux is mainly dominated by its ion surface exchange rate. Therefore, thinner membranes are desirable due to their higher solid state ionic transport rate than are thicker membranes. However, a lower ion surface exchange rate (i.e. a higher surface resistance to transport rate) of thinner membranes, becomes more dominating in the overall component transport rate. Surface resistances arise from various mechanisms involved in converting the molecules to be separated into ions or vice-versa at both surfaces of the membrane.
The prior art is replete with references and disclosures that involve the enhancement of the ion surface exchange rate by adding layers containing either porous dual phase mixed conductor coating or porous single phase mixed conductor coating onto a dense ceramic membrane material to enhance its flux.
For instance, in Y. Teraoka et al., "J. Ceram. Soc. Jpn. Inter. Ed.", Vol. 97, Nos. 4 and 5, pp. 523-529 (1989), discloses a gas separation membrane formed by depositing a dense mixed conducting oxide layer onto a porous mixed conducting support. In an example in which a suspension spray deposition technique was used to deposit the mixed conducting oxide layer, the resultant thin film element exhibited a two fold increase in oxygen permeation over a dense sintered sample without the deposited layer. Similarly, U.S. Pat. No. 5,240,480 discloses multi-layer composite solid state membranes exhibiting a superior oxygen flux that comprises a multicomponent metallic oxide porous layer and a dense layer.
U.S. Pat. No. 4,791,079 teaches an increased kinetic rate of the permeate side interfacial gas exchange through the use of a catalytic ceramic membrane consisting of two layers. The layers are an impervious mixed ion and electronic conducting ceramic layer and a porous catalyst-containing ion conducting ceramic layer. U.S. Pat. No. 5,723,035 illustrates the use of a porous coating of metal, metal oxide or combinations thereof to increase the kinetic rate of the feed side interfacial fluid exchange, the kinetic rate of the permeate side interfacial exchange, or both.
U.S. Pat. No. 5,938,822, discloses a membrane composed of a dense dual phase electronic-mixed conducting membrane and a porous dual phase electronic-mixed conductor or a porous single phase mixed conductor coating. The porous coating is disposed on at least one of the two opposed surfaces of the membrane to enhance the rate of surface reactions involving the gaseous species. This surface modification on the air side enhances ion surface exchange kinetics by increasing the surface area for oxygen dissociation.
It is apparent from the above discussion that all of the prior art membranes involve the fabrication of multi-layer membranes to improve ion surface exchange kinetics and therefore the oxygen permeation rate through the membrane. As will be discussed, the present invention employs a surface treatment to enhance membrane ion surface exchange kinetics that is less complicated in its execution than prior art techniques because it does not depend on the application of additional membrane layers.